Micro electro mechanical systems (MEMS) acoustic sensors (also referred to as MEMS microphones) are apparatuses for converting a voice to an electric signal, and the types of MEMS microphones includes a capacitive MEMS microphone and a piezoelectric MEMS microphone. The capacitive MEMS microphone uses a principle of a capacitor where two electrodes face each other, where one electrode is fixed onto a substrate and the other electrode is suspended in the air such that a vibrating plate reacts to an external acoustic pressure so as to be moved. That is, in the capacitive MEMS microphone, when the external acoustic pressure is input, the vibrating plate is vibrated and a capacitance value is changed while a gap between the two electrodes is changed, such that a current flows. Due to its stability and excellent frequency characteristics, such capacitive MEMS microphones are used for most of the existing MEMS microphones.
Meanwhile, in the MEMS microphone according to the related art, a rear acoustic chamber is formed by etching a fixed electrode in the form of a thin film and a lower substrate of the vibrating plate. In this case, the fixed electrode may be deformed, that is, deflected or bent by a residual stress or a pressure (for example, an acoustic pressure, an atmospheric pressure and the like) applied from the outside as an infrastructure supporting the fixed electrode at a lower portion thereof disappears. This may influence a performance of the MEMS microphone, and thus it is necessary to minimize deformation of the fixed electrode. In addition, since the vibrating plate needs to be maximally deformed in spite of an external acoustic pressure and the fixed electrode does not need to be deformed during an operation of the MEMS microphone to obtain high sensitivity, the fixed electrode does not need to be moved while having strength.
FIG. 1 is a view illustrating a structure of a bulk machined MEMS microphone according to the related art. FIG. 2 is a view illustrating a structure a surface machined MEMS microphone having a fixed electrode support structure.
Referring to FIG. 1, a vibrating plate 120 and a fixed electrode 130 are formed on a substrate 110, and a rear acoustic chamber 112 is formed to pass through a rear surface of the substrate 110 through bulk machining (U.S. Pat. No. 6,535,460).
Through this method, the rear acoustic chamber may be relatively simply formed on the rear surface of the substrate through bulk machining, but a fixed electrode support cannot be formed due to a structure which passes through the substrate.
In order to solve the problem, the fixed electrode may be made thick, but when the fixed electrode is made thick, air resistance of an exhaust hole of the fixed electrode increases, thereby deteriorating sensitivity. In addition, as an area of the fixed electrode increases, the fixed electrode becomes weaker to the deformation.
Referring to FIG. 2, a surface machined MEMS microphone structure is formed on a substrate 210, and a fixed electrode support 230 and a rear acoustic chamber 212 are formed by isotropic-etching the substrate 210 through an exhaust hole 222 formed in a lower fixed electrode 220 (IEEE SENSORS Conference 2009).
Unlike the above method, in this method, the fixed electrode support and the rear acoustic chamber are simultaneously formed by partially leaving a substrate functioning as a support under the fixed electrode by using an etching speed difference of the substrate according to the disposition of an exhaust hole.
However, the method is a method of forming the fixed electrode support by using a substrate itself, and can be implemented only when the fixed electrode directly contacts the substrate and cannot be implemented when the fixed electrode is located on the vibrating plate. In addition, if the vibrating plate is located on the fixed electrode, the vibrating plate may be directly exposed and damaged during a process, and may also be damaged due to the vibrating plate due to particles or moisture during an operation.
Thus, in order to protect the vibrating plate to obtain high sensitivity while increasing reliability in the surface machined MEMS microphone, the present disclosure provides a structure where the fixed electrode is disposed on the vibrating plate and the fixed electrode support is installed.